We have identified a unique signature for DNA polymerase zeta which is to generate tandem mutations during somatic hypermutation. Using mice deficient for various polymerases, we observed that polymerase eta is a dominant protein, and the effect of polymerase zeta was more pronounced in the absence of polymerase eta. Furthermore, data suggest that polymerase zeta functions in the MSH2-MSH6 pathway. An advantage to using polymerase zeta to introduce tandem mutations would be to efficiently change amino acids in variable genes to generate higher affinity antibodies. Similarly, we are determining if polymerase iota interacts with the replication fork to compete with polymerase zeta. A study of how proliferating cell nuclear antigen clamp unloaders interfere with cell division and class switch recombination and hypermutation is underway. We are also measuring single-strand breaks in variable genes to see how polymerases gain access to synthesize.